Here's my two cents on your problem:
1. If you're overweight, you need to start by cutting your body fat.
It may be (obviously I have no idea what you look like...) that you
unfortunately store body fat disporportionately in your hips/butt.
The only (nonsurgical) way to lose it is by losing fat. This is best
done by both a healthy diet and cardiovascular exercise.
2. The most important muscle group for making your shoulders look
wider (and your waist therefore narrower) are your latissimus dorsi
('lats'). These are the large muscles that pull your upper arms down
or back towards your body. Exercises like pull-ups (overhand), lat
pull downs, and upright rows are felt to be the best way to build size
and strength. The external obliques, like the other abdominal
muscles, are generally small and thin and don't lend themselves to
gaining bulk. Besides your lats, increasing the size of your deltoids
(your shoulders - strengthen by shoulder presses and lateral raises)
and your pectorals (your chest - push ups, bench presses, dumbell
flies) will help your upper body look larger than your waist.
Here's a picture of the lats - http://www.fitstep.com/Advanced/Anatomy/Back.htm
This site also has other diagrams of muscle groups.
To get you started on how to do exercises, check out
www.muscleandfitness.com/training and click on the Exercise Center
Hope this helps out.